1. Field of the Invention
This invention relates to the design of a hard disk drive (HDD) suspension that includes a slider mounted on a flexure. More particularly, it relates to a device and method using the device for measuring the flexure stiffness under load conditions.
2. Description of the Related Art
A hard disk drive (HDD) uses an encapsulated thin film magnetic read/write head (transducer), called a slider, to read and write data on a magnetic medium or storage disk. The slider has a pre-patterned air-bearing surface (ABS) and is a part of a flexible head gimbals assembly (HGA). Looking at FIG. 1, there is shown a highly schematic side view of a prior art HGA. The slider (10) is mounted on a gimbal assembly or flexure (20), The flexure is affixed to a relatively rigid loadbeam (30). The loadbeam exerts a downward force on the flexure through a downward pointing protrusion called a dimple (35). The loadbeam is connected to a base-plate (70) through a pivot or bend-zone (60). The combination of the loadbeam, the gimbal assembly (referred to herein as a flexure), the electrically conducting leads (or traces) (not shown), the pivot and a base-plate, is collectively termed the suspension. The suspension is activated by a servo actuator and associated electronic control circuitry to position the slider at various locations along the magnetically encoded tracks on the disk. As the disk is rapidly rotated by a spindle motor, hydrodynamic pressure causes an air flow between the ABS of the slider and the surface of the disk. This flow lifts and suspends the slider so that it literally flies above the surface of the disk (at a “fly height” of approximately 10 nm) on a layer of air called, appropriately, the air-bearing layer. The edge of the slider into which the disk rotates is called its “leading edge,” the opposite edge, which contains the read/write head is called the “trailing edge.” The loadbeam, as is known in the art, has a small downward extending protrusion or “dimple” ((35) in FIG. 1) formed on its disk-facing side that presses against the backside of the slider at a contact point, providing a downward force (typically 2.5 grams) and serving as a pivot for the slider to rotate about. This suspension system of loadbeam and gimbal provides mechanical support for the slider while also allowing the slider pitch and roll capability when fly height is achieved. In addition, the system provides an electrical connection (i.e., a placement for the routing of conducting traces) between the read/write head and the pre-amplifier.
In an operating disk drive the slider is “loaded” by its position over a rapidly spinning disk (i.e., the slider is placed under a combination of forces as a result of upward hydrodynamic pressure from the air bearing layer and mechanical downward forces). The downward component of the load force is due to elastic deformation of the suspension at the bend zone (shown in FIG. 1). This force is transmitted to the slider through the dimple that contacts the flexure.
The ABS of the slider is virtually parallel to the surface of an operationally spinning disk (well within 1 mrad of horizontal). However, when the slider is unloaded the ABS orientation is no longer a result of disk rotation and it deviates from its flying attitude. This deviation, referenced to the loadbeam orientation, is known as static attitude. It consists of two components, pitch static attitude (PSA) and roll static attitude (RSA). Corresponding to PSA, the flexure exhibits a pitch stiffness (kp). The product of PSA and kp represents a pitch moment exerted by the flexure on the flying slider.
Enabling the slider to fly in a stable manner above the disk places stringent requirements on the suspension design, such as providing a proper range of its vertical stiffness (Kz), gimbal (flexure) pitch and roll stiffness (Kp, Kr), gimbal pitch/roll static attitude (PSA/RSA), operational shock performance (G/gram) and the like. These requirements are mainly static and based on system geometry.
The flexure pitch stiffness increases when a load-force is applied through the loadbeam dimple to the slider. Traditional stiffness measurements are made in the absence of load forces. Such measurements are inaccurate for the purposes of slider air bearing surface (ABS) design, because the stiffness may increase significantly when the slider is loaded. Applying a load-force through mechanical contact is difficult, as any misalignment of the contact will affect the measurement. Unfortunately, there is no easy method to detect and correct such misalignment error. Similarly, friction at the ABS, caused when the load-force impedes slider motion in the plane of the ABS, also affects the measurement. This important friction term is statically indeterminate. Note that the physics of a flexure mounted slider under load, including the effects of dimple friction, is presented in “Slider Pitch Moment Associated with Dimple Friction” by Li-Yan Zhu, Chao-Hui Yang, Yen Fu and Ellis Cha, to be published in ASME J. Tribology, which is fully incorporated herein by reference.
A prior-art approach to measuring pitch and/or roll stiffness is to shake the HGA and measure the resulting slider resonance frequencies. Although a load-force similar to the “gram-load” present under actual operating conditions cannot be exerted, a small spring force, known as the dimple contact force (DCF), prevents dimple slip when the vibration amplitude is very small. Thus the “loaded” pitch stiffness can be calculated. This method, however, can only measure the pitch stiffness when the slider pitch is equal to its PSA. It cannot measure pitch stiffness at the flying condition, which is nearly zero pitch. Furthermore, this method does not fully reflect the contribution of dimple curvature to pitch and roll moments.
Another prior-art approach is to support the slider by two or more separate load cells. The nominal load force can be exerted by the load beam. The slider pitch and roll angles can be adjusted by translating at least one load cell. The pitch and roll moments can be calculated by multiplying the distance between two load cells by the difference in load forces exerted on the load cells. However, as described above, friction on the slider ABS is very difficult to assess and compensate.
The prior art does not disclose a significant number of methods for measuring flexure stiffness under the full gram load. The importance of stiffness, however, is well recognized in the prior art. Himes et al. (US Published Patent Application 2003/0007292) discusses the need for reducing stiffness at various points in the structure of the HGA. In particular, Himes teaches the fabrication of a low stiffness printed circuit interconnect to enable electrical connectivity within the HGA assembly. A method of measuring the stiffness of small assemblies, not specifically directed to flexures, is taught by Slocum et al. (US Published Patent Application 2003/0009898). Slocum teaches a method of applying a probe to an end of a flexible member and pushing the member while the displacement of the member is measured. Because of misalignment difficulties mentioned above, this method does not seem appropriate for measuring the stiffness of a flexure under actual loading conditions encountered during disk drive operation.
It is clear that a novel method is needed to overcome the shortcomings of the prior-art methodology and such a method will be presented herein.